lab_debug Disastrous Debugging
- Sunday, January 29 at 11:59 PM 1/29 @ 11:59 PM
Assignment Description
Learn how to debug your code!
Introduction to Debugging
Alice is writing an image recognition program. She’s working on the tracing
algorithm, which turns the image into a trace of the outlines in the image.
After going through all the compiler errors (sketchify.cpp:13, etc), the
program finally compiles! Overjoyed to have a program, she decides to test it
on a couple images.
Segmentation Fault
Ouch. What does she do now? She has to debug her code. Check out the code
To check out lab_debug, run
svn update
from your cs225 directory.
You should now have a directory named lab_debug that contains the needed code
for this lab.
If you don’t have SVN access yet, you can get the files by downloading
lab_debug.tar.gz.
Determining What’s Going Wrong
Alice could open sketchify.cpp and try to figure out what’s happening. This
is good for logical bugs — when you only rotate half of your image, for
example, or the image doesn’t rotate at all. Walking through what your code
does to yourself or your partner is a good exercise in debugging bugs in your
algorithm. However, this is often a poor choice for debugging runtime errors or
general code bugs. In this case, you should attempt to use the following
workflow to debug your code (taken mostly from “DEBUGGING: The 9 Indispensable
Rules for Finding Even the Most Elusive Software and Hardware Problems” by
David J. Agans):
Debugging Workflow
Understand the System.
- Without a solid understanding of the system (the system defined as being
both the actual machine you are running on as well as the general structure
behind the problem you are trying to solve), you can’t begin to narrow down
where a bug may be occurring. Start off by assembling knowledge of:
- What the task is
- What the code’s structure is
- What the control flow looks like
- How the program is accomplishing things (library usage, etc)
- When in doubt, look it up—this can be anything from using Google to find out what that system call does to simply reading through your lab’s code to see how it’s constructed.
- Without a solid understanding of the system (the system defined as being
both the actual machine you are running on as well as the general structure
behind the problem you are trying to solve), you can’t begin to narrow down
where a bug may be occurring. Start off by assembling knowledge of:
Make it Fail.
- The best way to understand why the bug is occurring is to make it happen again—in order to study the bug you need to be able to recreate it. And in order to be sure it’s fixed, you’ll have to verify that your fix works. In order to do that, you’ll need to have a reproducible test case for your bug.
- A great analogy here is to turn on the water on a leaky hose—only by doing that are you able to see where the tiny holes might be (and they may be obvious with the water squirting out of them!).
- You also need to fully understand the sequence of actions that happen up until the bug occurs. It could be specific to a certain type of input, for example, or only a certain branch of an if/else chain.
Quit Thinking and Look.
- After you’ve seen it fail, and seen it fail multiple times, you can generally have an idea of at least what function the program may be failing in. Use this to narrow your search window initially.
- Start instrumenting your code. In other words, add things that print out
intermediate values to check assumptions that should be true of variables
in your code. For instance, check that that pointer you have really is set
to
NULL. - Guessing initially is fine, but only if you’ve seen the bug before you attempt to fix it. Changing random lines of code won’t get you to a solution very fast, but will result in a lot of frustration (and maybe even more bugs)!
Divide and Conquer.
- Just like you’d use binary search on an array to find a number, do this on your code to find the offending line! Figure out whether you’re upstream of downstream of your bug: if your values look fine where you’ve instrumented, you’re upstream of the bug (it’s happening later on in the code). If the values look buggy, you’re probably downstream (the bug is above you).
- Fix the bugs as you find them—often times bugs will hide under one another. Fix the obvious ones as you see them on your way to the root cause.
Change One Thing at a Time.
- Use the scientific method here! Make sure that you only have one variable you’re changing at each step—otherwise, you won’t necessarily know what change was the one that fixed the bug (or whether or not your one addition/modification introduces more).
- What was the last thing that changed before it worked? If something was fine at an earlier version, chances are whatever you changed in the interim is what’s buggy.
Keep an Audit Trail.
- Keep track of what you’ve tried! This will prevent you from trying the same thing again, as well as give you an idea of the range of things you’ve tried changing.
Check the Plug.
- Make sure you’re assumptions are right! Things like “is my Makefile correct?” or “am I initializing everything?” are good things to make sure of before blindly assuming they’re right.
Get a Fresh View.
- Getting a different angle on the bug will often times result in a fix: different people think differently, and they may have a different perspective on your issue.
- Also, articulating your problem to someone often causes you to think about everything that’s going on in your control flow. You might even realize what is wrong as you are trying to describe it to someone! (This happens a lot during office hours and lab sections!)
- When talking to someone, though, make sure you’re sticking to the facts: report what is happening, but not what you think might be wrong (unless we’re asking you what you think’s going on).
If you didn’t fix it, it ain’t fixed.
- When you’ve got something you think works, test it! If you have a reproducible test case (you should if you’ve been following along), test it again. And test the other cases too, just to be sure that your fix of the bug didn’t break the rest of the program.
Basic Instrumentation: Print (cout) Statements!
The easiest way to debug your code is to add print statements. To do this, you can add comments at various points in your code, such as:
cout << "line " << __LINE__ << ": x = " << x << endl;
__LINE__ is a special compiler macro containing the current line number of
the file.
If you’re getting compiler errors after trying to use cout statements, then you
need to #include the iostream library like this:
#include <iostream>
using namespace std;
The above line prints out the current line number as well as the value of the
variable x when that line number executes, for example:
line 32: x = 3
Print statements work for debugging in (almost) any language and make repeated debug testing easy — to repeat debug testing with a new change, all you need to do is compile and run the program again. They also require nothing new to learn (smile) .
Debugging Alice’s Code
To make and run Alice’s code, type the following into your terminal:
make
cp in_01.png in.png
./sketchify
Alice’s First Bug
As you can see, Alice’s code caused a Segmentation Fault, or segfault. This
happens when you access memory that doesn’t belong to you — such as
dereferencing a NULL or uninitialized pointer.
Try adding print statements to lines 28 and 32, before and after the calls
to original->readFromFile(), width(), and height().
cout << "Reached line 28" << endl;
cout << "Reached line 32" << endl;
Now run sketchify again. You’ll see line 28 print out, but not line 32. This
means the segfault occurred sometime between executing lines 28 and 32. We’ll
let you figure out what the bug here is and how to fix it. You’ll see “line 32”
print to the terminal once you’ve finished — then move on to Bug 2, below.
Bug 2
Once you’ve fixed the first bug, you’ll get another segfault. You’ll want to
narrow down the line it’s occurring on and its cause by printing more
information. Try putting cout statements at the beginning and end of the
inner for loop.
More debugging!
Alice’s code, like most of ours, isn’t perfect. But fixing it is as simple as repeating the above to learn more about what the program is actually doing at runtime so that you can solve the issues. Good luck!
Checking Your Output
Once you think sketchify is working, you can compare your output (out.png) to
the expected output using diff:
diff out.png out_01.png
If they match (diff won’t give any output if they do), you’ve finished. If
not, you’ve still got a bit more debugging to do — go back and add some print
statements to figure out why the outputs differ.
If diff is unhelpful, you can open each image up using a graphical viewer:
xdg-open out.png &
xdg-open out_01.png &
Or compare them using compare:
compare out.png out_01.png comparison.png
xdg-open comparison.png &
Differences will be highlighted in red
Committing the Code
Commit your changes by using the following command:
svn commit -m "lab_debug submission"
Grading Information
The following files are used in grading:
sketchify.cpp
All other files including any testing files you have added will not be used for grading.